Unit 2: Study and Interpretation of Geological Maps

What is a Geological Map?

A geological map is a 2D representation of the 3D distribution of rocks and geological structures at the Earth's surface. It's a topographic map (showing hills, valleys, and contours) with geological information printed on top.

Key Components of a Geological Map:

• Topographic Base: Contour lines, rivers, roads, and other features.
• Geological Units: Different rock types or ages, shown as different colours and symbols (e.g., "Ss" for sandstone, "K" for Cretaceous).
• Contacts: Lines that separate different geological units.
• Strike and Dip Symbols: Show the orientation (attitude) of rock layers.
• Structural Symbols: Show the location of folds (anticlines, synclines) and faults.
• Legend/Key: Explains what all the colours and symbols mean, usually arranged in action (stratigraphic column) with the youngest rocks at the top.

Study and Interpretation of Maps

Interpreting a geological map means using the 2D information to understand the 3D geology and the geological history of the area.

The "Rule of V's"

This is the most important rule for interpreting maps. It describes how a planar feature (like a rock bed or a fault) interacts with topography (valleys).

• Horizontal Beds: Contacts are parallel to contour lines.
• Vertical Beds: Contacts run in uploads (straight lines) regardless of topography.
• Dipping Beds: Contacts form a "V" shape where they cross a valley.
  • The "V" points downstream if the bed is dipping upstream.
  • The "V" points upstream if the bed is dipping downstream (and the dip is gentler than the valley slope).
  • The sharper the "V", the gentler the dip.

Interpreting Geologic History:

By applying principles like Superposition and Cross-Cutting Relationships, you can read the story of the map.

1. Sedimentation: The rock layers (strata) were deposited (oldest at the bottom).
2. Deformation: The layers were tilted, folded (into anticlines/synclines), or faulted.
3. Intrusion: An igneous body (like a dike or batholith) cut through the older rocks.
4. Uplift and Erosion: The whole sequence was uplifted, and erosion (by rivers) carved the current landscape (topography).

Outcrop Completion

In a real-world map, rock outcrops (where the bedrock is visible) are often scattered and separated by soil, vegetation, or buildings (areas of "no exposure").

Outcrop completion is the process of intelligently "connecting the dots." It involves drawing the contact lines between the known outcrops based on the rules of geology.

How to Complete an Outcrop Map:

1. Identify Rock Units: Note the different rock types (e.g., Sandstone, Shale) at the known outcrops.
2. Find Strike and Dip: Use the strike/dip symbols (or the 3-point problem) to determine the orientation of the beds.
3. Draw Strike Lines: A strike line is a line of constant elevation on a dipping bed. You draw these lines (e.g., for 500m, 400m, 300m) across your map.
4. Connect the Contacts: The geological contact (the line separating two rock units) will be where its strike line intersects the matching topographic contour line.
5. Apply the Rule of V's: Draw the contact line by connecting these intersection points. Make sure it "V's" correctly in valleys and bends appropriately around hills.

Preparation of Cross Section of Geological Map

A geological cross-section is a 2D vertical "slice" into the Earth that shows what the geology looks like underground. It is the single most important interpretation of a geological map.

Step-by-Step Guide to Drawing a Cross-Section:

1. Choose a Line of Section: This is a line on the map (e.g., from point A to point B) where you want to see the "slice."
2. Prepare the Profile Paper:
   • Take a strip of paper. Lay it along the line of section (A-B).
   • Mark on the paper every point where a topographic contour line crosses your section line. Label each mark with its elevation (e.g., 500m, 400m).
   • Also, mark every point where a geological contact or fault crosses your line.
3. Draw the Topographic Profile:
   • On a piece of graph paper, draw a horizontal baseline (your line A-B) and a vertical scale (for elevation).
   • Transfer the marks from your paper strip onto the graph paper, plotting each point at its correct elevation.
   • Connect these points with a smooth curve. This is the topographic profile (the shape of the land).
4. Project the Geology:
   • Transfer the marks for the geological contacts straight down from the topographic profile onto your cross-section.
   • Now, you must draw the contacts *underground*. To do this, you need the dip angle of the beds.
   • Use a protractor to draw the dipping contact lines at their correct angle, starting from where they meet the surface.
5. Finalize the Section:
   • Color in the different rock units in your cross-section, matching the colors from the map.
   • Add a legend, title, and the horizontal and vertical scales (e.g., "Vertical Exaggeration = 1x").